DESCRIPTION: Squamous cell carcinoma (SCC) of the oral cavity is a debilitating and often fatal disease afflicting approximately 30,000 individuals annually in the United States and is a major health problem worldwide. To better understand the molecular biology governing the invasive and aggressive behavior of these tumors, differences in gene expression have been studied between non-malignant oral mucosa and SCC derived from the oral cavity. Using differential display reverse transcription-based PCR, a novel serine proteinase inhibitor (serpin) was cloned, called headpin, that is down regulated in SCC biopsies and in 50 percent of established head and neck squamous cell carcinoma (HNSCC) tumor lines. Headpin was mapped to a serpin cluster on chromosome 18q21, which is a region that often exhibits loss of heterozygosity in head and neck cancers. Purified and functional recombinant human headpin (rHeadpin) has been generated and kinetic analysis indicates it is a bona fide suicide-inhibitor of both cathepsin L (catL) and cathepsin K (catK). Immunohistochemistry using a highly specific mAb raised against headpin has confirmed that the protein is abundant in non-malignant oral epithelium and lost or down-regulated in primary and metastatic SCC from the oral cavity. Based on the current body of literature linking expression of catL to progression of tumors, the implicit role of catK in degrading bone extracellular matrix, and the ability of headpin to inhibit both of these enzymes, suggest that loss of headpin protein expression from oral SCCs contributes to their aggressive clinical behavior. To test this hypothesis, the full target spectrum of proteinases inhibited by headpin will be assessed, while investigating the in vitro and in vivo consequences of headpin re-expression in tumors, define the mechanism(s) of headpin loss in tumor specimens, and analyze the predictive clinical SIGNIFICANCE of headpin expression in archival specimens from patients with oral SCC. Progress in this area could lead to development of new molecular based targets for the management of oral cancer.